1. Field of the Invention
The present invention concerns an asynchronous motor power supply control system.
2. Description of the Prior Art
Asynchronous motors are usually supplied with power from an AC line voltage via a variable speed drive. The variable speed drive is a frequency converter including a rectifier bridge connected to the AC line voltage, a DC voltage intermediate circuit including a capacitor and an inverter including solid-state switches controlled by a pulse modulation device and connected to the phase windings of the motor.
When the supply of power to the motor is cut off the motor can be allowed to "free wheel". This retains control of the control device.
However, unless specific precautions are taken, resuming control of the motor speed "on the fly" can cause high current surges and momentary loss of speed. The current surges can damage the switches in the inverter.
Consequently, an object of the invention is to provide a system enabling resumption of motor speed control after cutting off the supply of power to the motor without causing current surges.
It is therefore necessary to determine the electrical speed .omega. and to set the angular position of the voltage vector relative to the motor flux at the time the power supply returns. The voltage vector is a mathematical representation of the resultant of the actual voltages applied to the motor and the electrical speed is the product of the mechanical speed of the motor by the number of poles.
The object as stated is achieved by means for controlled resumption of motor speed control such as an identification structure which estimates the electrical speed of the motor and sets the angular position of the voltage vector in a rotating orthogonal system of axes (d, q) in which the (d) axis corresponds to the motor flux.
The identification structure advantageously includes a structure for estimating the speed based on an angle calculation unit which determines the angular position of the voltage vector by calculating its angle .alpha. to an orthogonal system of axes (.alpha., .beta.) which is fixed relative to the motor stator and a differentiator unit which differentiates the angle .alpha..
The angle calculation unit determines the coordinates V.sub..alpha. and V.sub..beta. of the voltage vector in the fixed system of axes (.alpha., .beta.) to deduce therefrom the angle .alpha..
A positioning unit sets the angular position of the voltage vector in phase quadrature to the estimated motor flux.
The means for controlled resumption of motor speed control also include a device for progressively augmenting the component V.sub.q of the voltage up to a control value subject to a time constant which is dependent on the rotor time constant.
The system can include a cascade regulation structure for limiting the current if the evolution of the estimated voltage towards the control voltage of the motor is faster than the rotor time constant.
The following description with reference to the drawings will highlight the features and advantages of one specific embodiment of the invention.